In the case of using a nonaqueous electrolytic solution such as electrolytic solution for plating with a metal ion unstable in an aqueous solution system, electrolytic solution for lithium and other batteries, and electrolytic solution for capacitor, it is very important to remove impurities in the nonaqueous electrolytic solution. In these uses, the water amount in the nonaqueous electrolytic solution needs to be 50 ppm or less. Therefore, in use as a nonaqueous electrolytic solution, the solution must be previously subjected to a dehydration treatment.
Among others, in a lithium secondary battery, when water is present in the nonaqueous electrolytic solution, not only the negative electrode performance of the battery is reduced but also decomposition of the electrolyte salt in the nonaqueous electrolytic solution is accelerated. Therefore, removal of water in the nonaqueous electrolytic solution is a very important task.
Examples of the dehydration treatment method of the nonaqueous electrolytic solution, which has been heretofore proposed, include a method of separately drying a nonaqueous solvent and an electrolyte and then mixing both to prepare a nonaqueous electrolytic solution, a method of azeotropically dehydrating a mixture of a nonaqueous solvent and an electrolyte (Patent Document 1), a method of dehydrating a mixture of a nonaqueous solvent and an electrolyte by a zeolite (Patent Document 2), and a method comprising a combination thereof (Patent Document 3). These dehydration treatment methods are technically classified roughly into two groups, that is, 1) a method of performing dehydration of a nonaqueous electrolytic solution by distillation or drying, and 2) a method of performing dehydration of a nonaqueous electrolytic solution by using a zeolite.
The method of 1) includes a method of separately drying a nonaqueous solvent and an electrolyte and then mixing both to prepare a nonaqueous electrolytic solution, and a method of azeotropically dehydrating a nonaqueous electrolytic solution in the state of an electrolyte being dissolved in a nonaqueous solvent. In the former case, water is liable to be mixed in the course of mixing the nonaqueous solvent and the electrolyte, whereas in the azeotropic dehydration of the latter case, it is difficult to sufficiently remove water in the nonaqueous electrolytic solution. Therefore, in both methods, the water amount in the nonaqueous electrolytic solution can be hardly reduced to 50 ppm or less.
The method of 2) is a method of removing water in the nonaqueous electrolytic solution by utilizing water adsorption capacity of a zeolite. However, an ion-exchangeable cation is present in the zeolite, and lithium ion in the nonaqueous electrolytic solution and the cation in the zeolite cause an ion exchange reaction during the dehydration treatment. Therefore, in this method, although water in the nonaqueous electrolytic solution may be removed, a cation in the zeolite elutes as an impurity into the nonaqueous electrolytic solution and contaminates the nonaqueous electrolytic solution after removal of water.
As a technique to solve this problem, a method of previously ion-exchanging the ion-exchangeable cation in the zeolite with a cation which does not become a contamination source, for example, in the case of a nonaqueous electrolytic solution for lithium battery, a method of ion-exchanging the zeolite with a cation except for sodium, has been proposed (Patent Documents 2, 4 and 5). However, although the ion-exchangeable cation is previously exchanged with lithium, in the case of dehydrating the nonaqueous electrolytic solution by using a lithium substitution-type zeolite where sodium remains, the ion-exchangeable cation in the zeolite is not completely ion-exchanged with lithium ion and therefore, the problem of causing elution of the sodium ion into the nonaqueous electrolytic solution from the zeolite cannot be avoided (Patent Document 3). On the other hand, for completely ion-exchanging the ion-exchangeable cation in the zeolite with lithium, a very large amount of high-purity lithium is required. In turn, a zeolite where the ion-exchangeable cation is completely exchanged with lithium ion is expensive.
In addition, a method of keeping the electrolytic solution from contacting with the zeolite for a long time and thereby suppressing the ion exchange reaction between the cation in the zeolite and the ion in the electrolytic solution (Patent Document 3) has been proposed as the method for dehydrating the nonaqueous electrolytic solution by using a zeolite. However, in such a dehydration method, the process is complicated.
In this way, despite the attempts to enhance the dehydration capacity in the method for dehydrating the nonaqueous electrolytic solution by using a zeolite, conventional methods have a problem of sodium elution from the zeolite or a problem of the complicated process. For this reason, there is not known a zeolite for treatment of a nonaqueous electrolytic solution, which involves no sodium elution and enables dehydration treatment of the nonaqueous electrolytic solution by a simple process.